Mobile communication system and user terminal

ABSTRACT

When a UE 100-2 (a reception-side UE) that performs D2D communication by using UL radio resource switches communication modes between cellular communication and the D2D communication, if DL subframe in which data reception of the cellular communication should be performed and the UL subframe in which data reception of the D2D communication should be performed at least partially overlap on a time axis, then the UE 100-2 performs data reception in one subframe, out of the overlapping two subframes.

TECHNICAL FIELD

The present invention relates to a mobile communication system and auser terminal configured to support D2D communication.

BACKGROUND ART

In 3GPP (3rd Generation Partnership Project) which is a project aimingto standardize a mobile communication system, the introduction of Deviceto Device (D2D) communication is discussed as a new function afterRelease 12 (see Non Patent Document 1).

In the D2D communication, a plurality of adjacent user terminals performdirect device-to-device communication without passing through a corenetwork. On the other hand, a data path of normal communication(cellular communication) of a mobile communication system passes throughthe core network.

In the D2D communication, it is supposed that an uplink radio resourceor a downlink radio resource of cellular communication is used. Theuplink radio resource includes a plurality of uplink subframes dividedon a time axis. The downlink radio resource includes a plurality ofdownlink subframes divided on a time axis.

PRIOR ART DOCUMENT Non-Patent Document

Non Patent Document 1:3GPP technical report “TR 22.803 V12.1.0” March,2013

SUMMARY OF THE INVENTION

In cellular communication, it is common that because of a propagationdelay between a user terminal and a base station and control tocompensate for the delay, for example, an uplink subframe and a downlinksubframe handled by the user terminal are inconsistent on a time axis.

Accordingly, in a case where the uplink radio resource is used for D2Dcommunication, when communication modes are switched between thecellular communication and the D2D communication, a situation may occurthat the downlink subframe in which data reception of the cellularcommunication should be performed and the uplink subframe in which datareception of the D2D communication should be performed at leastpartially overlap on a time axis (that is, simultaneous reception ofcellular and D2D).

Here, the user terminal does not necessarily have a capability toperform simultaneous reception of cellular and D2D. Alternatively, evenin a case that the user terminal has the capability to perform thesimultaneous reception of cellular and D2D, if the simultaneousreception of cellular and D2D is performed, then a communication qualitydeteriorates due to interference.

Therefore, to resolve such a problem, when the communication modes areswitched between the cellular communication and the D2D communication,in a case that the downlink subframe in which the data reception of thecellular communication should be performed and the uplink subframe inwhich the data reception of the D2D communication should be performedoverlap on a time axis, a method may be considered where the both of theoverlapping two subframes are not used.

However, in such a method, there is a problem that it is not possible toeffectively use a radio resource because the radio resource by the twosubframes are useless.

On the other hand, in a case where the downlink radio resource is usedfor the D2D communication, when the communication modes are switchedbetween the cellular communication and the D2D communication, asituation may occur that the uplink subframe in which the datatransmission of the cellular communication should be performed and thedownlink subframe in which the data transmission of the D2Dcommunication should be performed at least partially overlap on a timeaxis (that is, simultaneous transmission of cellular and D2D).

Therefore, in a case where the downlink radio resource is used for theD2D communication, a problem occurs similarly to the case where theuplink radio resource is used for the D2D communication.

Therefore, an object of the present invention is to provide a mobilecommunication system and a user terminal with which it is possible toeffectively utilize a radio resource when communication modes areswitched between cellular communication and D2D communication and toavoid simultaneous reception of cellular and D2D or simultaneoustransmission of cellular and D2D.

In a mobile communication system according to a first aspect, an uplinkradio resource of cellular communication includes a plurality of uplinksubframes divided on a time axis and a downlink radio resource of thecellular communication includes a plurality of downlink subframesdivided on a time axis. The mobile communication system comprises: auser terminal that performs D2D communication that is directdevice-to-device communication by using the uplink radio resource. Whenthe user terminal switches communication modes between the cellularcommunication and the D2D communication, if the downlink subframe inwhich data reception of the cellular communication should be performedand the uplink subframe in which data reception of the D2D communicationshould be performed at least partially overlap on a time axis, then theuser terminal performs data reception in one subframe, out of theoverlapping two subframes.

In a mobile communication system according to a second aspect, an uplinkradio resource of cellular communication includes a plurality of uplinksubframes divided on a time axis and a downlink radio resource of thecellular communication includes a plurality of downlink subframesdivided on a time axis. The mobile communication system comprises: auser terminal configured to perform D2D communication that is directdevice-to-device communication by using the downlink radio resource.When the user terminal switches communication modes between the cellularcommunication and the D2D communication, if the uplink subframe in whichdata transmission of the cellular communication should be performed andthe downlink subframe in which data transmission of the D2Dcommunication should be performed at least partially overlap on a timeaxis, the user terminal performs the data transmission in one subframe,out of the overlapping two subframes.

A user terminal according to a third aspect is configured to perform D2Dcommunication that is direct device-to-device communication by using anuplink radio resource in a mobile communication system in which theuplink radio resource of cellular communication includes a plurality ofuplink subframes divided on a time axis, and a downlink radio resourceof the cellular communication includes a plurality of downlink subframesdivided on a time axis. The user terminal comprises: a controllerconfigured to perform, when communication modes are switched between thecellular communication and the D2D communication, if the downlinksubframe in which data reception of the cellular communication should beperformed and the uplink subframe in which data reception of the D2Dcommunication should be performed at least partially overlap on a timeaxis, data reception in one subframe, out of the overlapping twosubframes.

A user terminal according to a fourth aspect is configured to performD2D communication that is direct device-to-device communication by usinga downlink radio resource in a mobile communication system in which anuplink radio resource of cellular communication includes a plurality ofuplink subframes divided on a time axis and the downlink radio resourceof the cellular communication includes a plurality of downlink subframesdivided on a time axis. The user terminal comprises: a controllerconfigured to perform, when communication modes are switched between thecellular communication and the D2D communication, if the uplink subframein which data transmission of the cellular communication should beperformed and the downlink subframe in which data transmission of theD2D communication should be performed at least partially overlap on atime axis, a data transmission in one subframe, out of the overlappingtwo subframes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a configuration diagram of an LTE system according to a firstembodiment to a third embodiment.

FIG. 2 is a block diagram of UE according to the first embodiment to thethird embodiment.

FIG. 3 is a block diagram of eNB according to the first embodiment tothe third embodiment.

FIG. 4 is a protocol stack diagram of a radio interface according to thefirst embodiment to the third embodiment.

FIG. 5 is a configuration diagram of a radio frame according to thefirst embodiment to the third embodiment.

FIG. 6 is a diagram for describing D2D communication according to thefirst embodiment to the third embodiment.

FIG. 7 is a diagram for describing a switching operation from cellularcommunication to D2D communication in a case of performing D2Dcommunication by using a UL radio resource, in the first embodiment.

FIG. 8 is a diagram for describing the switching operation from thecellular communication to the D2D communication in a case of performingthe D2D communication by using the UL radio resource, in the firstembodiment.

FIG. 9 is a diagram for describing a detail of the switching operationfrom the cellular communication to the D2D communication in a case ofperforming the D2D communication by using the UL radio resource, in thefirst embodiment.

FIG. 10 is a sequence diagram showing the switching operation from thecellular communication to the D2D communication in a case of performingthe D2D communication by using the UL radio resource, in the firstembodiment.

FIG. 11 is a flow diagram showing an operation of eNB in the sequence ofFIG. 10.

FIG. 12 is a diagram for describing a switching operation from D2Dcommunication to cellular communication in a case of performing D2Dcommunication by using a UL radio resource, in the first embodiment.

FIG. 13 is a diagram for describing a detail of the switching operationfrom D2D communication to cellular communication in a case of performingD2D communication by using a UL radio resource, in the first embodiment.

FIG. 14 is a diagram for describing the switching operation from thecellular communication to the D2D communication in a case of performingthe D2D communication by using a DL radio resource, in the firstembodiment.

FIG. 15 is a diagram for describing a detail of the switching operationfrom the cellular communication to the D2D communication in a case ofperforming the D2D communication by using the DL radio resource, in thefirst embodiment.

FIG. 16 is a diagram for describing a switching operation from D2Dcommunication to cellular communication in a case of performing D2Dcommunication by using a DL radio resource, in the first embodiment.

FIG. 17 is a diagram for describing a detail of the switching operationfrom D2D communication to cellular communication in a case of performingD2D communication by using a DL radio resource, in the first embodiment.

FIG. 18 is a diagram for describing an operation according to the secondembodiment.

FIG. 19 is a diagram for describing format identification informationindicating a setting format for a switching guard time according to thesecond embodiment to the third embodiment.

FIG. 20 is a sequence diagram showing a switching operation fromcellular communication to D2D communication in a case of performing D2Dcommunication by using a UL radio resource in the second embodiment.

FIG. 21 is a flow diagram showing an operation of eNB 200 in thesequence in FIG. 20.

FIG. 22 is a diagram for describing an operation according to the thirdembodiment.

FIG. 23 is a sequence diagram showing the switching operation from thecellular communication to the D2D communication in a case of performingthe D2D communication by using a UL radio resource, in the thirdembodiment.

DESCRIPTION OF THE EMBODIMENT

[Overview of Embodiment]

In a mobile communication system according to a first embodiment to athird embodiment, an uplink radio resource of cellular communicationincludes a plurality of uplink subframes divided on a time axis and adownlink radio resource of the cellular communication includes aplurality of downlink subframes divided on a time axis. The mobilecommunication system comprises: a user terminal that performs D2Dcommunication that is direct device-to-device communication by using theuplink radio resource. When the user terminal switches communicationmodes between the cellular communication and the D2D communication, ifthe downlink subframe in which data reception of the cellularcommunication should be performed and the uplink subframe in which datareception of the D2D communication should be performed at leastpartially overlap on a time axis, then the user terminal performs datareception in one subframe, out of the overlapping two subframes.

In a first embodiment, the mobile communication system furthercomprises: a base station configured to assign a radio resource to theuser terminal. When the user terminal switches communication modesbetween the cellular communication and the D2D communication, if thedownlink subframe in which data reception of the cellular communicationshould be performed and the uplink subframe in which data reception ofthe D2D communication should be performed at least partially overlap ona time axis, the base station assigns the radio resource correspondingto the one subframe, to the user terminal, and assigns the radioresource corresponding to the other subframe, out of the overlapping twosubframes, to a user terminal other than the user terminal.

In a second embodiment, the other subframe, out of the overlapping twosubframes, includes a non-overlapping interval not overlapping, on atime axis, the one subframe. The user terminal further performs datareception in the non-overlapping interval included in the othersubframe.

In a second embodiment, the mobile communication system furthercomprises: a base station configured to assign a radio resource to theuser terminal. When the user terminal switches communication modesbetween the cellular communication and the D2D communication, if thedownlink subframe in which data reception of the cellular communicationshould be performed and the uplink subframe in which data reception ofthe D2D communication should be performed at least partially overlap ona time axis, then the base station assigns the radio resourcecorresponding to the one subframe, to the user terminal, and assigns theradio resource corresponding to the non-overlapping interval included inthe other subframe, to the user terminal.

In a third embodiment, the other subframe, out of the overlapping twosubframes, includes a non-overlapping interval not overlapping, on atime axis, the one subframe. The user terminal performs a discoveryprocess of the D2D communication in the non-overlapping intervalincluded in the other subframe.

In a third embodiment, the mobile communication system furthercomprises: a base station configured to assign a radio resource to theuser terminal. When the user terminal switches communication modesbetween the cellular communication and the D2D communication, if thedownlink subframe in which data reception of the cellular communicationshould be performed and the uplink subframe in which data reception ofthe D2D communication should be performed at least partially overlap ona time axis, then the base station assigns the radio resourcecorresponding to the one subframe, to the user terminal, and assigns theradio resource corresponding to the non-overlapping interval included inthe other subframe, to the user terminal for the discovery process.

In a mobile communication system a first embodiment to a thirdembodiment, an uplink radio resource of cellular communication includesa plurality of uplink subframes divided on a time axis and a downlinkradio resource of the cellular communication includes a plurality ofdownlink subframes divided on a time axis. The mobile communicationsystem comprises: a user terminal configured to perform D2Dcommunication that is direct device-to-device communication by using thedownlink radio resource. When the user terminal switches communicationmodes between the cellular communication and the D2D communication, ifthe uplink subframe in which data transmission of the cellularcommunication should be performed and the downlink subframe in whichdata transmission of the D2D communication should be performed at leastpartially overlap on a time axis, the user terminal performs the datatransmission in one subframe, out of the overlapping two subframes.

In a first embodiment, the mobile communication system furthercomprises: a base station configured to assign the radio resource to theuser terminal. When the user terminal switches communication modesbetween the cellular communication and the D2D communication, if theuplink subframe in which data transmission of the cellular communicationshould be performed and the downlink subframe in which data transmissionof the D2D communication should be performed at least partially overlapon a time axis, then the base station assigns the radio resourcecorresponding to the one subframe, to the user terminal, and assigns theradio resource corresponding to the other subframe, out of theoverlapping two subframes, a user terminal other than the user terminal.

In a second embodiment, the other subframe, out of the overlapping twosubframes, includes a non-overlapping interval not overlapping, on atime axis, the one subframe. The user terminal further performs a datatransmission in the non-overlapping interval included in the othersubframe.

In a second embodiment, the mobile communication system furthercomprises: a base station configured to assign the radio resource to theuser terminal. When the user terminal switches communication modesbetween the cellular communication and the D2D communication, if theuplink subframe in which data transmission of the cellular communicationshould be performed and the downlink subframe in which data transmissionof the D2D communication should be performed at least partially overlapon a time axis, then the base station assigns the radio resourcecorresponding to the one subframe, to the user terminal, and assigns theradio resource corresponding to the non-overlapping interval included inthe other subframe, to the user terminal.

In a third embodiment, the other subframe, out of the overlapping twosubframes, includes a non-overlapping interval not overlapping, on atime axis, the one subframe. The user terminal performs a discoveryprocess of the D2D communication in the non-overlapping intervalincluded in the other subframe.

In a third embodiment, the mobile communication system furthercomprises: a base station configured to assign a radio resource to theuser terminal. When the user terminal switches communication modesbetween the cellular communication and the D2D communication, if theuplink subframe in which data transmission of the cellular communicationshould be performed and the downlink subframe in which data transmissionof the D2D communication should be performed at least partially overlapon a time axis, then the base station assigns the radio resourcecorresponding to the one subframe, to the user terminal, and assigns theradio resource corresponding to the non-overlapping interval included inthe other subframe, to the user terminal for the discovery process.

A user terminal according to a first embodiment to a third embodiment isconfigured to perform D2D communication that is direct device-to-devicecommunication by using an uplink radio resource in a mobilecommunication system in which the uplink radio resource of cellularcommunication includes a plurality of uplink subframes divided on a timeaxis, and a downlink radio resource of the cellular communicationincludes a plurality of downlink subframes divided on a time axis. Theuser terminal comprises: a controller configured to perform, whencommunication modes are switched between the cellular communication andthe D2D communication, if the downlink subframe in which data receptionof the cellular communication should be performed and the uplinksubframe in which data reception of the D2D communication should beperformed at least partially overlap on a time axis, data reception inone subframe, out of the overlapping two subframes.

A user terminal according to a first embodiment to a third embodiment isconfigured to perform D2D communication that is direct device-to-devicecommunication by using a downlink radio resource in a mobilecommunication system in which an uplink radio resource of cellularcommunication includes a plurality of uplink subframes divided on a timeaxis and the downlink radio resource of the cellular communicationincludes a plurality of downlink subframes divided on a time axis. Theuser terminal comprises: a controller configured to perform, whencommunication modes are switched between the cellular communication andthe D2D communication, if the uplink subframe in which data transmissionof the cellular communication should be performed and the downlinksubframe in which data transmission of the D2D communication should beperformed at least partially overlap on a time axis, a data transmissionin one subframe, out of the overlapping two subframes.

[First Embodiment]

Hereinafter, a description will be provided for an embodiment in a casewhere the present invention is applied to an LTE system.

(System Configuration)

FIG. 1 is a configuration diagram of an LTE system according to thefirst embodiment. As illustrated in FIG. 1, the LTE system includes UEs(User Equipment) 100, E-UTRAN (Evolved-UMTS Terrestrial Radio AccessNetwork) 10, and EPC (Evolved Packet Core) 20.

The UE 100 corresponds to a user terminal. The UE 100 is a mobilecommunication device and performs radio communication with a connectingcell (serving cell). A configuration of the UE 100 will be describedbelow in detail.

The E-UTRAN 10 corresponds to a radio access network. The E-UTRAN 10includes eNBs 200 (evolved Node-B). The eNB 200 corresponds to a basestation. The eNBs 200 are connected to one another via an X2 interface.A configuration of the eNB 200 will be described below in detail.

Each eNB 200 manages one or a plurality of cells and performs radiocommunication with the UE 100 which establishes a connection with thecell of the eNB 200. The eNB 200, for example, has a radio resourcemanagement (RRM) function, a routing function of user data, and ameasurement control function for mobility control and scheduling. The“cell” is used as a term indicating a minimum unit of a radiocommunication area, and is also used as a term indicating a function ofperforming radio communication with the UE 100.

The EPC 20 corresponds to a core network. The E-UTRAN 10 and the EPC 20configure a network of the LTE system. The EPC 20 includes a pluralityof MME (Mobility Management Entity)/S-GWs (Serving-Gateway) 300. The MMEperforms various mobility controls and the like for the UE 100. The S-GWperforms transfer control of user data. The MME/S-GW 300 is connected tothe eNBs 200 via an S1 interface.

FIG. 2 is a block diagram of the UE 100. As illustrated in FIG. 2, theUE 100 includes an antenna 101, a radio transceiver 110, a userinterface 120, a GNSS (Global Navigation Satellite System) receiver 130,a battery 140, a memory 150, and a processor 160. The memory 150 and theprocessor 160 configure a controller. The UE 100 may not have the GNSSreceiver 130. Furthermore, the memory 150 may be integrally formed withthe processor 160, and this set (that is, a chip set) may be called aprocessor 160′.

The antenna 101 and the radio transceiver 110 are used to transmit andreceive a radio signal. The radio transceiver 110 converts a basebandsignal (transmission signal) output from the processor 160 into theradio signal, and transmits the radio signal from the plurality ofantennas 101. Furthermore, the radio transceiver 110 converts the radiosignal received by the plurality of antennas 101 into the basebandsignal (reception signal), and outputs the baseband signal to theprocessor 160.

The user interface 120 is an interface with a user carrying the UE 100,and includes, for example, a display, a microphone, a speaker, andvarious buttons. The user interface 120 receives an operation from auser and outputs a signal indicating the content of the operation to theprocessor 160. The GNSS receiver 130 receives a GNSS signal in order toobtain location information indicating a geographical location of the UE100, and outputs the received signal to the processor 160. The battery140 accumulates a power to be supplied to each block of the UE 100.

The memory 150 stores a program to be executed by the processor 160 andinformation to be used for a process by the processor 160. The processor160 includes a baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signal,and a CPU (Central Processing Unit) that performs various processes byexecuting the program stored in the memory 150. The processor 160 mayfurther include a codec that performs encoding and decoding on sound andvideo signals. The processor 160 executes various processes and variouscommunication protocols, which will be described later.

FIG. 3 is a block diagram of the eNB 200. As illustrated in FIG. 3, theeNB 200 includes an antenna 201, a radio transceiver 210, a networkinterface 220, a memory 230, and a processor 240. The memory 230 and theprocessor 240 constitute a controller.

The antenna 201 and the radio transceiver 210 are used to transmit andreceive a radio signal. The radio transceiver 210 converts a basebandsignal (transmission signal) output from the processor 240 into theradio signal, and transmits the radio signal from the plurality ofantennas 201. Furthermore, the radio transceiver 210 converts the radiosignal received by the plurality of antennas 201 into the basebandsignal (reception signal), and outputs the baseband signal to theprocessor 240.

The network interface 220 is connected to the neighboring eNB 200 viathe X2 interface and is connected to the MME/S-GW 300 via the S1interface. The network interface 220 is used in communication performedon the X2 interface and communication performed on the S1 interface.

The memory 230 stores a program to be executed by the processor 240 andinformation to be used for a process by the processor 240. The processor240 includes the baseband processor that performs modulation anddemodulation, encoding and decoding and the like on the baseband signaland a CPU that performs various processes by executing the programstored in the memory 230. The processor 240 executes various processesand various communication protocols, which will be described later.

FIG. 4 is a protocol stack diagram of a radio interface in the LTEsystem. As illustrated in FIG. 4, the radio interface protocol isclassified into a layer 1 to a layer 3 of an OSI reference model,wherein the layer 1 is a physical (PHY) layer. The layer 2 includes anMAC (Medium Access Control) layer, an RLC (Radio Link Control) layer,and a PDCP (Packet Data Convergence Protocol) layer. The layer 3includes an RRC (Radio Resource Control) layer.

The PHY layer performs encoding and decoding, modulation anddemodulation, antenna mapping and demapping, and resource mapping anddemapping. Between the PHY layer of the UE 100 and the PHY layer of theeNB 200, user data and control signal are transmitted via the physicalchannel.

The MAC layer performs priority control of data, and a retransmissionprocess and the like by hybrid ARQ (HARQ). Between the MAC layer of theUE 100 and the MAC layer of the eNB 200, user data and control signalare transmitted via a transport channel. The MAC layer of the eNB 200includes a transport format of an uplink and a downlink (a transportblock size and a modulation and coding scheme) and a scheduler fordetermining a resource block to be assigned to the UE100.

The RLC layer transmits data to an RLC layer of a reception side byusing the functions of the MAC layer and the PHY layer. Between the RLClayer of the UE 100 and the RLC layer of the eNB 200, user data andcontrol signal are transmitted via a logical channel.

The PDCP layer performs header compression and decompression, andencryption and decryption.

The RRC layer is defined only in a control plane dealing with a controlsignal. Between the RRC layer of the UE 100 and the RRC layer of the eNB200, a control signal (an RRC message) for various types of setting istransmitted. The RRC layer controls the logical channel, the transportchannel, and the physical channel in response to establishment,re-establishment, and release of a radio bearer. When there is anconnection (RRC connection) between the RRC of the UE 100 and the RRC ofthe eNB 200, the UE 100 is in a connected state (an RRC connectedstate), and when there is no connection (no RRC connection), the UE 100is in an idle state (an RRC idle state).

An NAS (Non-Access Stratum) layer positioned above the RRC layerperforms session management, mobility management and the like.

FIG. 5 is a configuration diagram of a radio frame used in the LTEsystem. In the LTE system, OFDMA (Orthogonal Frequency DivisionMultiplexing Access) is applied to a downlink (DL), and SC-FDMA (SingleCarrier Frequency Division Multiple Access) is applied to an uplink(UL), respectively.

As illustrated in FIG. 5, the radio frame is configured by 10 subframesarranged in a time direction. Each subframe is configured by two slotsarranged in the time direction. Each subframe has a length of 1 ms andeach slot has a length of 0.5 ms. Each subframe includes a plurality ofresource blocks (RB) in a frequency direction, and a plurality ofsymbols in the time direction. Each resource block includes a pluralityof subcarriers in the frequency direction. A radio resource unit isconfigured by one subcarrier and one symbol and one subcarrier.

Among radio resources allocated to the UE 100, a frequency resource canbe configured by a resource block and a time resource can be configuredby a subframe (or slot).

In the DL, an interval of several symbols from the head of each subframeis a region used as a physical downlink control channel (PDCCH) formainly transmitting a control signal. Furthermore, the other portion ofeach subframe is a region available as a physical downlink sharedchannel (PDSCH) for mainly transmitting user data.

In the UL, both ends in the frequency direction of each subframe areregions used as a physical uplink control channel (PUCCH) for mainlytransmitting a control signal. The central portion of each subframe is aregion available as a physical uplink shared channel (PUSCH) for mainlytransmitting user data.

(Operation according to first embodiment) An LTE system according to thefirst embodiment supports D2D communication that is directdevice-to-device communication (UE-to-UE communication). Here, the D2Dcommunication is described in comparison with cellular communicationthat is normal communication of the LTE system. The cellularcommunication is a communication mode in which a data path passesthrough a network (E-UTRAN 10, EPC 20). The data path is a communicationpath for user data. On the other hand, the D2D communication is acommunication mode in which a data path set between UEs does not passthrough the network.

FIG. 6 is a diagram for describing the D2D communication.

As shown in FIG. 6, in the D2D communication, a data path does not passthrough the eNB 200. A UE 100-1 and a UE 100-2 adjacent to each otherdirectly perform radio communication with low transmission power in acell of the eNB 200. The UE 100-1 is a transmission-side UE configuredto perform transmission of user data (hereinafter, simply called “datatransmission”). The UE 100-2 is a reception-side UE configured toreceive user data (hereinafter, simply called “data reception”).

Thus, when the adjacent UE 100-1 and UE 100-2 directly perform radiocommunication with low transmission power, it is possible to reduce apower consumption of the UE 100 and to reduce interference to aneighbouring cell, in comparison with in the cellular communication. Inthe D2D communication, it is supposed that a UL radio resource (ULbandwidth) or a DL radio resource (DL bandwidth) of the cellularcommunication is used. The UL radio resource includes a plurality of ULsubframes divided on a time axis. The DL radio resource includes aplurality of DL subframes divided on a time axis.

Below, an operation according to the first embodiment will be describedin order of (1) a case of performing the D2D communication by using theUL radio resource, and (2) a case of performing the D2D communication byusing the DL radio resource.

(1) Case of Performing D2D Communication by Using UL Radio Resource

In the cellular communication, it is common that because of apropagation delay between the UE 100 and the eNB 200 and control tocompensate for the delay, for example, the UL subframe and the DLsubframe handled by the UE 100 are inconsistent on a time axis. Thus, ina case of using the UL radio resource for the D2D communication, whencommunication modes are switched between the cellular communication andthe D2D communication, a situation may occur that the DL subframe inwhich data reception of the cellular communication should be performedand the UL subframe in which data reception of the D2D communicationshould be performed overlap (that is, simultaneous reception of cellularand D2D) at least partially on a time axis. The UE 100 does notnecessarily have a capability to perform the simultaneous reception ofcellular and D2D. Alternatively, even in a case that the user terminalhas the capability to perform the simultaneous reception of cellular andD2D, if the simultaneous reception of cellular and D2D is performed,then a communication quality deteriorates due to interference.

Therefore, in the first embodiment, when the communication modes areswitched between the cellular communication and the D2D communication,in a case that the DL subframe in which the data reception of thecellular communication should be performed and the UL subframe in whichthe data reception of the D2D communication should be performed overlapat least partially on a time axis, the UE 100-2 (reception-side UE)configured to perform the D2D communication by using the UL radioresource performs the data reception in one subframe, out of theoverlapping two subframes.

Further, when the UE 100-2 switches the communication modes between thecellular communication and the D2D communication, in a case that the DLsubframe in which the data reception of the cellular communicationshould be performed and the UL subframe in which the data reception ofthe D2D communication should be performed overlap at least partially ona time axis, the eNB 200 configured to assign the radio resource to theUE 100-2 assigns the radio resource corresponding to one subframe, tothe UE 100-2, and assigns the radio resource corresponding to the othersubframe, out of the overlapping two subframes, to the UE 100 (UE 100-X)other than the UE 100-2.

Therefore, in a case of performing the D2D communication by using the ULradio resource, when the communication modes are switched between thecellular communication and the D2D communication, it is possible toeffectively utilize the radio resource and avoid the simultaneousreception of cellular and D2D.

Below, a case of performing the D2D communication by using the UL radioresource will be described in order of (1.1) a switching operation fromthe cellular communication to the D2D communication, and (1.2) aswitching operation from the D2D communication to the cellularcommunication.

(1.1) Switching Operation from Cellular Communication to D2DCommunication

FIG. 7 and FIG. 8 are diagrams for describing the switching operationfrom the cellular communication to the D2D communication.

As shown in FIG. 7, in an initial state, each of the UE 100-1 and the UE100-2 performs the cellular communication (the UL communication and theDL communication) in the cell of the eNB 200. Here, an operationenvironment is supposed where the UE 100-1 and the UE 100-2 are locatedfar from the eNB 200.

In the DL communication, when a delay time passes by a propagation delayafter the eNB 200 transmits the user data, the UE 100-1 and the UE 100-2receive the user data. Thus, the DL subframe in the UE 100-1 and the UE100-2 is located, on a time axis, after the DL subframe in the eNB 200.

On the other hand, in the UL communication, control to compensate forthe propagation delay is applied. Specifically, the eNB 200 transmits atiming advance value for adjusting a transmission timing to each of theUE 100-1 and the UE 100-2. To the timing advance value, a valueindicating an adjustment amount to the present transmission timing isset so that a reception timing in the eNB 200 reaches a target timing.Each of the UE 100-1 and the UE 100-2 adjusts the transmission timing onthe basis of the timing advance value received from the eNB 200. Thus,the UL subframe in the UE 100-1 and the UE 100-2 are located before, ona time axis, the UL subframe in the eNB 200.

As a result, the UL subframe handled by each of the UE 100-1 and the UE100-2 is located before, on a time axis, the DL subframe.

In such a situation, in a case that the UE 100-1 and the UE 100-2 switchfrom the cellular communication to the D2D communication, in the UE100-2 (reception-side UE), a DL subframe F11 in which the data receptionof the cellular communication should be performed and a UL subframe F12in which the data reception of the D2D communication should be performedoverlap on a time axis, resulting in a situation where a simultaneousreception of cellular and D2D is performed. In FIG. 7, a rear-sideportion of the DL subframe F11 in which the data reception of thecellular communication should be performed overlaps a front-side portionof the UL subframe F12 in which the data reception of the D2Dcommunication should be performed.

Thus, as shown in FIG. 8, the eNB 200 configured to assign a radioresource to the UE 100-2 assigns a radio resource corresponding to onesubframe, out of the overlapping two subframes, to the UE 100-2 andassigns a radio resource corresponding to the other subframe, out of theoverlapping two subframes, to the UE 100 (UE 100-X) other than the UE100-2.

For example, the eNB 200 assigns, to the UE100-2, the radio resourcecorresponding to the UL subframe F12 in which the data reception of theD2D communication should be performed, out of the overlapping twosubframes. On the other hand, the eNB 200 does not assign the radioresource corresponding to the DL subframe F11 in which the datareception of the cellular communication should be performed to the UE100-2, out of the overlapping two subframes, but assigns to another UE100-X the radio resource. The other UE 100-X is the UE 100 configured toperform the cellular communication with the eNB 200.

Alternatively, the eNB 200 may assign, to the UE 100-2, the radioresource corresponding to the DL subframe F11 in which the datareception of the cellular communication should be performed, out of theoverlapping two subframes. In this case, the eNB 200 may not assign, tothe UE 100-2, the radio resource corresponding to the UL subframe F12 inwhich the data reception of the D2D communication should be performed,out of the overlapping two subframes, but may assign the radio resourceto the other UE 100-X.

FIG. 9 is a diagram for describing a detail of the switching operationfrom the cellular communication to the D2D communication.

As shown in FIG. 9, in the UE 100-2, the UL subframe is located before,on a time axis, the DL subframe. Here, a case is supposed where thecellular communication is performed until a UL subframe “UL3” and a DLsubframe “DL3” then switched to the D2D communication. In this case,when the UE 100-2 performs the data reception of the D2D communicationin the UL subframe “UL4”, the result is that a contention occurs withthe data reception of the cellular communication (DL communication) inthe DL subframe “DL3”.

Thus, the eNB 200 does not assign to the UE 100-2 the radio resourcecorresponding to the UL subframe “UL4”. In other words, the eNB 200 andthe UE 100-2 set the UL subframe “UL4” as a switching guard time.

Then, the eNB 200 assigns to the UE 100-2 the radio resourcecorresponding to each UL subframe after the UL subframe “UL5”. The UE100-2 performs the data reception of the D2D communication in each ULsubframe after the UL subframe “UL5”.

FIG. 10 is a sequence diagram showing the switching operation from thecellular communication to the D2D communication. In FIG. 10, a situationis supposed where the UE 100-1 and the UE 100-2 are adjacent to eachother, and UE 100-1 and the UE 100-2 perform the cellular communicationtherebetween through a network.

As shown in FIG. 10, in step S101, each of the UE 100-1, the UE 100-2,and UE 100-X performs the D2D communication with the eNB 200. The eNB200 assigns the radio resource for the cellular communication to each ofthe UE 100-1, the UE 100-2, and the UE 100-X.

In step S102, the eNB 200 determines that the UE 100-1 and the UE 100-2are switched from the cellular communication to the D2D communication.For example, in response to either one of the UE 100-1 or the UE 100-2finding the other, the eNB 200 determines to switch from the cellularcommunication to the D2D communication. Alternatively, the eNB 200 maydetermine on the basis of location information of each of the UE 100-1and the UE 100-2 to switch from the cellular communication to the D2Dcommunication in response to the eNB 200 detecting that the UE 100-1 andthe UE 100-2 are adjacent to each other. Alternatively, the eNB 200 maydetermine on the basis of CSI (Channel State Information) fed back fromthe UE 100-1 or the UE 100-2 to switch from the cellular communicationto the D2D communication.

In step S103, the eNB 200 notifies, in unit of subframe, the UE 100-1and the UE 100-2 of a switching timing from the cellular communicationto the D2D communication. In an example of FIG. 9, the UE 100-1 and theUE 100-2 are notified that the cellular communication is performed untilthe “UL3” and the “DL3”, and the cellular communication is switched tothe D2D communication after the “UL3” and the “DL3”. It is noted thatstep S103 may be performed simultaneously with step S106 describedlater. Alternatively, when the eNB 200 assigns a radio resourceassignment for the D2D communication, step S103 may be performedsimultaneously with the assignment.

In step S104, the eNB 200 confirms whether or not the UE 100-2 that is areception-side UE in the D2D communication supports simultaneousreception of cellular and D2D. For example, the eNB 200 confirms on thebasis of capability notification information (UE Capability) receivedfrom the UE 100-2 in advance whether or not the UE 100-2 supports thesimultaneous reception of cellular and D2D. In this case, the capabilitynotification information (UE Capability) includes information indicatingwhether or not the simultaneous reception of cellular and D2D issupported.

In a case that the UE 100-2 does not support the simultaneous receptionof cellular and D2D, in step S105, the eNB 200 calculates the switchingguard time in unit of subframe on the basis of the timing advance valueset to the UE 100-2. For example, the eNB 200 acquires an accumulatedvalue of the timing advance value, and determines on the basis of theaccumulated value how far the UL subframe and the DL subframe overlap inthe UE 100-2. It is noted that in the first embodiment, the switchingguard time is in unit of subframe, and thus, such a severe determinationis not necessarily necessary.

In step S106, the eNB 200 notifies the UE 100-1 and the UE 100-2 of theswitching guard time in unit of subframe. In an example of FIG. 9, theeNB 200 notifies the UE 100-1 and the UE 100-2 of the “UL4” as theswitching guard time.

In step S107, the eNB 200 determines assignment of a radio resource tothe UE 100-1, the UE 100-2, and UE 100-X. In an example of FIG. 9, theeNB 200 determines to not assign the radio resource corresponding to the“UL4” set as the switching guard time, to the UE 100-1 and the UE 100-2,but assign the radio resource to UE 100-X.

In step S108, the eNB 200 transmits assignment information indicatingthe radio resource corresponding to the switching guard time, to the UE100-X. The UE 100-X utilizes the radio resource for the cellularcommunication. In an example of FIG. 9, the UE 100-X utilizes the radioresource corresponding to the “UL4” for the cellular communication (ULcommunication).

In step S109, the UE 100-1 and UE 100-2 start the D2D communicationafter the switching guard time passes. In an example of FIG. 9, the UE100-1 performs the data transmission in the “UL5”, and the UE 100-2performs the data reception in the “UL5”.

FIG. 11 is a flow diagram showing an operation of the eNB 200 in thesequence of FIG. 10. The present flow corresponds to step S104 and S105in FIG. 10. It is noted that the determination in the present flow maybe omitted partially.

As shown in FIG. 11, in step S151, the eNB 200 confirms whether or notthe UE 100-2 that is a reception-side UE in the D2D communicationsupports the simultaneous reception of cellular and D2D.

In a case that the UE 100-2 supports the simultaneous reception ofcellular and D2D (step S151: No), the eNB 200 determines in step S152that there is no switching guard time.

On the other hand, in a case that the UE 100-2 does not support thesimultaneous reception of cellular and D2D (step S151: Yes), the eNB 200confirms in step 153 whether or not the timing advance value(accumulated value) set to the UE 100-2 is equal to or more than zero.

In a case that the timing advance value (accumulated value) set to theUE 100-2 is less than zero (step S153: No), the eNB 200 determines instep 154 that the switching guard time is zero (that is, no switchingguard time).

On the other hand, in a case that the timing advance value (accumulatedvalue) set to the UE 100-2 is more than zero (step S153: Yes), the eNB200 determines in step 155 that the switching guard time is one subframe(that is, 1 ms).

(1.2) Switching Operation from D2D Communication to CellularCommunication

Next, the switching operation from the D2D communication to the cellularcommunication will be described. It is noted that the description of theoperation overlapping the aforementioned operation will be omitted.

FIG. 12 is a diagram for describing the switching operation from the D2Dcommunication to the cellular communication.

As shown in FIG. 12, in an initial state, the UE 100-1 and the UE 100-2perform the D2D communication in a cell of the eNB 200. Here, anoperation environment is supposed where the UE 100-1 and the UE 100-2are located in the vicinity of the eNB 200.

In such an operation environment, contrary to the operation environmentwhere the UE 100-1 and the UE 100-2 are located far from the eNB 200,adjustment is made so that the UL subframe timing is delayed by thetiming advance value. That is, the accumulated value of the timingadvance value is less than zero (negative value). As a result, the ULsubframe handled by each of the UE 100-1 and the UE 100-2 is located, ona time axis, after the DL subframe.

In such a situation, in a case that the UE 100-1 and the UE 100-2 switchfrom the D2D communication to the cellular communication, in the UE100-2 (reception-side UE), a DL subframe F21 in which the data receptionof the cellular communication should be performed and a UL subframe F22in which the data reception of the D2D communication should be performedoverlap on a time axis, resulting in a situation where the simultaneousreception of cellular and D2D is performed. In FIG. 12, a rear-sideportion of the UL subframe F22 in which the data reception of the D2Dcommunication should be performed overlaps a front-side portion of theDL subframe F21 in which the data reception of the cellularcommunication should be performed.

Thus, the eNB 200 configured to assign the radio resource to the UE100-2 assigns a radio resource corresponding to one subframe, out of theoverlapping two subframes, to the UE 100-2 and assigns a radio resourcecorresponding to the other subframe, out of the overlapping twosubframes, to the UE 100 (UE 100-X) other than the UE 100-2.

FIG. 13 is a diagram for describing a detail of the switching operationfrom the D2D communication to the cellular communication.

As shown in FIG. 13, in the UE 100-2, the DL subframe is located before,on a time axis, the UL subframe. Here, a case is supposed where the D2Dcommunication is performed until the UL subframe “UL3” then switched tothe cellular communication. In this case, when the UE 100-2 performs thedata reception of the D2D communication in the “UL3”, the result is thata contention occurs with the data reception of the cellularcommunication (DL communication) in the DL subframe “DL4”.

Thus, the eNB 200 does not assign to the UE 100-2 the radio resourcecorresponding to the DL subframe “DL4”. In other words, the eNB 200 andthe UE 100-2 set the DL subframe “DL4” as the switching guard time.

Then, the eNB 200 assigns to the UE 100-2 the radio resourcecorresponding to each DL subframe after a DL subframe “DL5”. The UE100-2 performs the DL communication in each DL subframes after the“DL5”.

(2) Case of Performing D2D Communication by Using DL Radio Resource

In a case of using the DL radio resource for the D2D communication, whenthe communication modes are switched between the cellular communicationand the D2D communication, a situation may occur that the UL subframe inwhich data reception of the cellular communication should be performedand the DL subframe in which data reception of the D2D communicationshould be performed overlap (that is, simultaneous transmission ofcellular and D2D) at least partially on a time axis. Here, the UE 100does not necessarily have a capability to perform the simultaneoustransmission of cellular and D2D. Alternatively, even in a case that theUE 100 has the capability to perform the simultaneous transmission ofcellular and D2D, if the UE 100 performs the simultaneous transmissionof cellular and D2D, then a communication quality deteriorates due tointerference.

Therefore, in the first embodiment, when the communication modes areswitched between the cellular communication and the D2D communication,in a case that the UL subframe in which the data transmission of thecellular communication should be performed and the DL subframe in whichthe data transmission of the D2D communication should be performedoverlap at least partially on a time axis, the UE 100-1(transmission-side UE) configured to perform the D2D communication byusing the DL radio resource receives data performs the data transmissionin one subframe, out of the overlapping two subframes.

Further, when the UE 100-1 switches the communication modes between thecellular communication and the D2D communication, in a case that the ULsubframe in which the data transmission of the cellular communicationshould be performed and the DL subframe in which the data transmissionof the D2D communication should be performed overlap at least partiallyon a time axis, the eNB 200 configured to assign the radio resource tothe UE 100-1 assigns the radio resource corresponding to one subframe,to the UE100-1, and assigns a radio resource corresponding to the othersubframe, out of the overlapping two subframes, to the UE 100 (UE 100-X)other than the UE 100-1.

Therefore, in a case of utilizing the D2D radio resource for the D2Dcommunication, when the communication modes are switched between thecellular communication and the D2D communication, it is possible toeffectively utilize the radio resource and avoid the simultaneoustransmission of cellular and D2D.

Below, a case of performing the D2D communication by using the DL radioresource communication will be described in order of (2.1) switchingoperation from the cellular communication to the D2D communication, and(2.2) switching operation from the D2D communication to the cellularcommunication. It is noted that the description of the operationoverlapping the aforementioned operation will be omitted.

(2.1) Switching Operation from Cellular Communication to D2DCommunication

FIG. 14 is a diagram for describing the switching operation from thecellular communication to the D2D communication.

As shown in FIG. 14, in an initial state, each of the UE 100-1 and theUE 100-2 performs the cellular communication (the UL communication andthe DL communication) in a cell of the eNB 200. Here, an operationenvironment is supposed where the UE 100-1 and the UE 100-2 are locatedin the vicinity of the eNB 200. In such an operation environment, the ULsubframe handled by each of the UE 100-1 and the UE 100-2 is located, ona time axis, after the DL subframe.

In such a situation, in a case that the UE 100-1 and the UE 100-2 switchfrom the cellular communication to the D2D communication, in the UE100-1 (transmission-side UE), a UL subframe F32 in which the datatransmission of the cellular communication should be performed and a DLsubframe F31 in which the data transmission of the D2D communicationshould be performed overlap on a time axis, resulting in a situationwhere the simultaneous transmission of cellular and D2D is performed. InFIG. 14, a rear-side portion of the UL subframe F32 in which the datatransmission of the cellular communication should be performed overlapsfront-side portion of the DL subframe F31 in which the data transmissionof the D2D communication should be performed overlap.

Thus, the eNB 200 configured to assign the radio resource to the UE100-1 assigns a radio resource corresponding to one of the overlappingtwo subframes to the UE 100-1 and assigns a radio resource correspondingto the other subframe of the overlapping two subframes to the UE 100other than the UE 100-1 (UE 100-X).

For example, the eNB 200 assigns to the UE 100-1 the radio resourcecorresponding to the DL subframe F31 in which the data transmission ofthe D2D communication should be performed, out of the overlapping twosubframes. On the other hand, the eNB 200 does not assign to the UE100-1 the radio resource corresponding to the UL subframe F32 in whichthe data transmission of the cellular communication should be performed,out of the overlapping two subframes, but assigns the radio resource toanother UE 100-X.

Alternatively, the eNB 200 may assign to the UE 100-1 the radio resourcecorresponding to the UL subframe F32 in which the data transmission ofthe cellular communication should be performed, out of the overlappingtwo subframes. In this case, the eNB 200 may not assign to the UE 100-1the radio resource corresponding to the DL subframe F31 in which thedata transmission of the D2D communication should be performed, out ofthe overlapping two subframes, but may assign the radio resource toanother UE 100-X.

FIG. 15 is a diagram for describing a detail of the switching operationfrom the cellular communication to the D2D communication.

As shown in FIG. 15, in the UE 100-1, the DL subframe is located before,on a time axis, the UL subframe. Here, a case is supposed where thecellular communication is performed until the UL subframe “UL3” thenswitched to the D2D communication. In this case, when the UE 100-1performs the data transmission of the cellular communication in the ULsubframe “UL4”, which is next to the UL subframe “UL3”, the result isthat a contention occurs with the data transmission of the D2Dcommunication in the DL subframe “DL5”.

Thus, the eNB 200 does not assign to the UE 100-1 the radio resourcecorresponding to the UL subframe “UL4”. In other words, the eNB 200 andthe UE 100-1 set the UL subframe “UL4” as the switching guard time.

Then, the eNB 200 assigns to the UE 100-1 the radio resourcecorresponding to each DL subframes after the DL subframe “DL5”. The UE100-1 performs the data transmission of the D2D communication in each DLsubframe after the DL subframe “DL5”.

(2.2) Switching Operation from D2D Communication to CellularCommunication

Next, the switching operation from the D2D communication to the cellularcommunication will be described. It is noted that the description of theoperation overlapping the aforementioned operation will be omitted.

FIG. 16 is a diagram for describing the switching operation from the D2Dcommunication to the cellular communication.

As shown in FIG. 16, in an initial state, the UE 100-1 and the UE 100-2perform the D2D communication in a cell of the eNB 200. Here, anoperation environment is supposed where the UE 100-1 and the UE 100-2are located far from the eNB 200. In such an operation environment,adjustment is made so that the UL subframe timing is advanced by thetiming advance value. That is, the accumulated value of the timingadvance value is a positive value. As a result, the DL subframe handledby each of the UE 100-1 and the UE 100-2 is located, on a time axis,after the UL subframe.

In such a situation, in a case that the UE 100-1 and the UE 100-2 switchfrom the D2D communication to the cellular communication, in the UE100-1 (transmission-side UE), a UL subframe F42 in which the datatransmission of the cellular communication should be performed and a DLsubframe F41 in which the data transmission of the D2D communicationshould be performed overlap on a time axis, resulting in a situationwhere the simultaneous transmission of cellular and D2D is performed. InFIG. 16, a rear-side portion of the DL subframe F41 in which the datatransmission of the D2D communication should be performed overlaps afront-side portion of the DL subframe F42 in which the data transmissionof the cellular communication should be performed.

Thus, the eNB 200 configured to assign the radio resource to the UE100-1 assigns a radio resource corresponding to one of the overlappingtwo subframes to the UE 100-1 and assigns a radio resource correspondingto the other subframe of the overlapping two subframes to the UE 100other than the UE 100-1 (UE 100-X).

FIG. 17 is a diagram for describing a detail of the switching operationfrom the D2D communication to the cellular communication.

As shown in FIG. 17, in the UE 100-1, the UL subframe is located before,on a time axis, the DL subframe. Here, a case is supposed where the D2Dcommunication is performed until the DL subframe “DL3” then switched tothe cellular communication. In this case, when the UE 100-1 performs thedata transmission of the D2D communication in the “DL3”, the result isthat a contention occurs with the data transmission of the cellularcommunication (UL communication) in the UL subframe “UL4”.

Thus, the eNB 200 does not assign to the UE 100-1 the radio resourcecorresponding to the UL subframe “UL4”. In other words, the eNB 200 andthe UE 100-1 set the UL subframe “UL4” as the switching guard time.

Then, the eNB 200 assigns to the UE 100-1 the radio resourcecorresponding to each UL subframe after the UL subframe “UL5” and theradio resource corresponding to each DL subframe after the DL subframe“DL5”. The UE 100-1 performs the UL communication in each UL subframeafter the UL subframe “UL5” and performs the DL communication in each DLsubframe after the DL subframe “DL5”.

[Second Embodiment]

In a second embodiment, description proceeds with a particular focus ona difference from the first embodiment.

In the above-described first embodiment, the switching guard time is setin unit of subframe, and in the second embodiment, the switching guardtime is set in unit of symbol.

In the second embodiment, in a case of performing the D2D communicationby using the UL radio resource, the other subframe, out of theoverlapping two subframes, includes a non-overlapping interval having nooverlapping with one subframe on a time axis. The UE 100-2(reception-side UE) further performs the data reception in thenon-overlapping interval included in the other subframe. The eNB 200configured to assign the radio resource to the UE 100-2 assigns theradio resource corresponding to one subframe, to the UE 100-2, andassigns the radio resource corresponding to the non-overlapping intervalincluded in the other subframe, to the UE 100-2. As a result, when thecommunication modes are switched between the cellular communication andthe D2D communication, it is possible to further enhance a utilizationefficiency of the radio resource and avoid the simultaneous reception ofcellular and D2D.

In the second embodiment, in a case of performing the D2D communicationby using the DL radio resource, the other subframe, out of theoverlapping two subframes, includes the non-overlapping interval havingno overlapping with one subframe on a time axis. The UE 100-1(transmission-side UE) further performs the data transmission in thenon-overlapping interval included the other subframe. The eNB 200configured to assign the radio resource to the UE 100-1 assigns theradio resource corresponding to one subframe, to the UE100-1, andassigns the radio resource corresponding to the non-overlapping intervalincluded in the other subframe, to the UE100-1. As a result, when thecommunication modes are switched between the cellular communication andthe D2D communication, it is possible to further enhance a utilizationefficiency of the radio resource and avoid the simultaneous transmissionof cellular and D2D.

FIG. 18 is a diagram for describing an operation according to the secondembodiment. Here, in a case of performing the D2D communication by usingthe UL radio resource, the switching operation from the cellularcommunication to the D2D communication will be described. It is notedthat it is possible to apply this feature to the switching operationfrom the D2D communication to the cellular communication and also to acase that the D2D communication is performed by using the DL radioresource.

As shown in FIG. 18, in an initial state, each of the UE 100-1 and theUE 100-2 performs the cellular communication (the UL communication andthe DL communication) in a cell of the eNB 200. Here, an operationenvironment is supposed where the UE 100-1 and the UE 100-2 are locatedfar from the eNB 200. In such an operation environment, the UL subframethe UL subframe handled by each of the UE 100-1 and the UE 100-2 islocated before, on a time axis, the DL subframe.

In such a situation, in a case that the UE 100-1 and the UE 100-2 switchfrom the cellular communication to the D2D communication, in the UE100-2 (reception-side UE), a situation occurs that a DL subframe FM inwhich the data reception of the cellular communication should beperformed and a UL subframe F52 in which the data reception of the D2Dcommunication should be performed overlap on a time axis, resulting in asituation where the simultaneous cellular and D2D reception isperformed. In FIG. 18, a rear-side portion of the DL subframe FM inwhich the data reception of the cellular communication should beperformed overlaps a front-side portion of the DL subframe F52 in whichthe data reception of the D2D communication should be performed.

Thus, in the second embodiment, the eNB 200 configured to assign theradio resource to the UE 100-2 assigns the radio resource correspondingto one subframe, out of the overlapping two subframes, to the UE 100-2,and assigns the radio resource corresponding to the non-overlappinginterval included in the other subframe, to the UE 100-2.

In FIG. 18, the eNB 200 assigns to the UE 100-2 the radio resourcecorresponding to the DL subframe F51 in which the data reception of thecellular communication should be performed, out of the overlapping twosubframes. Further, the eNB 200 assigns, in unit of symbol, to the UE100-2 the radio resource corresponding to the non-overlapping intervalincluded in the UL subframe F52 in which the data reception of the D2Dcommunication should be performed, out of the overlapping two subframes.

As a result, in the UL subframe F52 in which the data reception of theD2D communication should be performed, the overlapping intervaloverlapping the DL subframe in which the data reception of the cellularcommunication should be performed is set as the switching guard time(interval with no data). Further, in the UL subframe F52 in which thedata reception of the D2D communication should be performed, thenon-overlapping interval not overlapping the DL subframe F51 in whichthe data reception of the cellular communication should be performed isset as an interval where the data reception is performed.

It is noted that to notify the UE 100-1 and the UE 100-2 of theswitching guard time, the eNB 200 may transmit to the UE 100-1 and theUE 100-2 symbol identification information indicating each symbolconfiguring the switching guard time, and may also transmit to the UE100-1 and the UE 100-2 format identification information indicating asetting format for the switching guard time.

FIG. 19 is a diagram for describing the format identificationinformation indicating a setting format for the switching guard time.

As shown in FIG. 19, a plurality of setting formats of the switchingguard time are defined in advance. A format 0 is a format where there isno switching guard time. In an example of FIG. 19, a format 1 is aformat in which a head symbol of the subframe is designated as theswitching guard time. A format 2 is a format in which the head symbol ofthe subframe and a second symbol thereof are designated as the switchingguard time. A format 3 is a format in which the head symbol of thesubframe to a third symbol thereof are designated as the switching guardtime. It is noted that FIG. 19 provides just an example, and the numberof symbols in the switching guard time may be defined as any number.

FIG. 20 is a sequence diagram showing the switching operation from thecellular communication to the D2D communication in a case of performingthe D2D communication by using the UL radio resource. In FIG. 20, asituation is supposed where the UE 100-1 and the UE 100-2 are adjacentto each other and the UE 100-1 and the UE 100-2 perform the cellularcommunication therebetween through a network. Further, in FIG. 20, anexample is described where the switching from the cellular communicationto the D2D communication is determined by the UE 100-1; however, theswitching may be determined by the eNB 200.

As shown in FIG. 20, in step S201, each of the UE 100-1 and the UE 100-2performs the cellular communication with the eNB 200. The eNB 200assigns to each of the UE 100-1 and the UE 100-2 the radio resource forthe cellular communication.

In step S202, the UE 100-1 determines to switch from the cellularcommunication to the D2D communication. For example, the UE 100-1determines to switch from the cellular communication to the D2Dcommunication in response to the UE 100-2 being discovered.

In step S203, the UE 100-1 transmits to the eNB 200 a switching requestto request a switch from the cellular communication to the D2Dcommunication.

In step S204, the eNB 200 configured to receive the switching requesttransmits to the UE 100-1 a switching permission to permit a switch fromthe cellular communication to the D2D communication. It is noted thatstep S204 may be performed simultaneously with step S205 or S208described later.

In step S205, the eNB 200 notifies, in unit of subframe, the UE 100-1and the UE 100-2 of the switching timing from the cellular communicationto the D2D communication. It is noted that S205 may be performedsimultaneously with step S208 described later.

In step S206, the eNB 200 confirms whether or not the UE 100-2 that is areception-side UE in the D2D communication supports the simultaneousreception of cellular and D2D.

When the UE 100-2 does not support the simultaneous reception ofcellular and D2D, in step S207, the eNB 200 calculate, in unit ofsymbol, the switching guard time on the basis of the timing advancevalue set to the UE 100-2. For example, the eNB 200 acquires anaccumulated value of the timing advance value, and determines on thebasis of the accumulated value how far the UL subframe and the DLsubframe overlap in the UE 100-2.

In step S208, the eNB 200 notifies, in unit of symbol, the UE 100-1 andthe UE 100-2 of the switching guard time. As described above, it ispossible to notify the switching guard time by an index of the format.

In step S209, the eNB 200 performs thinning-out of the user data inaccordance with the switching guard time to reduce the number of dataassignment symbols.

In step S210, the eNB 200 transmits to the UE 100-1 the user data thatis thinned-out.

In step S211, the UE 100-1 and the UE 100-2 starts the D2D communicationafter the switching guard time passes.

FIG. 21 is a flow diagram showing an operation of the eNB 200 in thesequence shown in FIG. 20. The present flow corresponds to step S206 andS207 in FIG. 20. It is noted that the determination in the present flowmay be omitted partially.

As shown in FIG. 21, in step S251, the eNB 200 confirms whether or notthe UE 100-2 that is a reception-side UE in the D2D communicationsupports the simultaneous reception of cellular and D2D.

When the UE 100-2 supports the simultaneous reception of cellular andD2D (step S251: No), the eNB 200 determines in step S252 that there isno switching guard time.

On the other hand, when the UE 100-2 does not support the simultaneousreception of cellular and D2D (step S251: Yes), in step 253, the eNB 200confirms whether or not the timing advance value (accumulated value) setto the UE 100-2 is equal to or more than zero.

When the timing advance value (accumulated value) set to the UE 100-2 isless than zero (step S253: No), in step 254, the eNB 200 determines thatthe switching guard time is zero (that is, there is no switching guardtime).

On the other hand, when the timing advance value set to the UE 100-2(accumulated value) is equal to or more than zero (step S253: Yes), instep 255, the eNB 200 confirms whether or not the remainder obtained bydividing the timing advance value (accumulated value) by a symbol lengthis zero.

When “Yes” is determined in step S255, in step S256, the eNB 200designates the value obtained by dividing the timing advance value(accumulated value) by the symbol length as the number of symbols in theswitching guard time. As a result, it is possible to set the switchingguard time having a time length corresponding to the overlappinginterval.

On the other hand, when “No” is determined in step S255, in step S257,the eNB 200 designates, as the number of symbols in the switching guardtime, a result obtained by adding “1” to the value obtained by dividingthe timing advance value (accumulated value) by the symbol length. As aresult, it is possible to set the switching guard time having a timelength covering the overlapping interval.

[Third Embodiment]

In a third embodiment, description proceeds with a particular focus on adifference from the first embodiment and the second embodiment.

The third embodiment is in much the same way as in the second embodimentin that the switching guard time is set in unit of symbol. It is notedthat the third embodiment differs from the second embodiment in that aninterval other than the switching guard time is used for transmitting orreceiving a discovery-use signal for the D2D communication (hereinafter,called “Discovery signal”) in the subframe in which the switching guardis arranged.

In the third embodiment, in a case of performing the D2D communicationby using the UL radio resource, the other subframe, out of theoverlapping two subframes, includes a non-overlapping interval notoverlapping, on a time axis, the one subframe. The UE 100-2(reception-side UE) further transmits the Discovery signal in thenon-overlapping interval included in the one subframe. The eNB 200configured to assign the radio resource to the UE 100-2 assigns theradio resource corresponding to one subframe, to the UE 100-2, andassigns, to the UE 100-2, the radio resource corresponding to thenon-overlapping interval included in the other subframe for transmittingor receiving the Discovery signal. As a result, when the communicationmodes are switched between the cellular communication and the D2Dcommunication, it is possible to further enhance a utilizationefficiency of the radio resource and avoid the simultaneous reception ofcellular and D2D.

In the third embodiment, in a case of performing the D2D communicationby using the DL radio resource, the other subframe, out of theoverlapping two subframes, includes the non-overlapping interval notoverlapping, on a time axis, the one subframe. The UE 100-1(transmission-side UE) further transmits or receives the Discoverysignal in the non-overlapping interval included in the other subframe.The eNB 200 configured to assign the radio resource to the UE 100-1assigns the radio resource corresponding to one subframe, to theUE100-1, and assigns, to the UE100-1, the radio resource correspondingto the non-overlapping interval included in the other subframe fortransmitting or receiving the Discovery signal. As a result, when thecommunication modes are switched between the cellular communication andthe D2D communication, it is possible to further enhance a utilizationefficiency of the radio resource and avoid the simultaneous transmissionof cellular and D2D.

FIG. 22 is a diagram for describing an operation according to the thirdembodiment. Here, in a case of performing the D2D communication by usingthe UL radio resource, the switching operation from the cellularcommunication to the D2D communication will be described. It is notedthat it is possible to apply this notification to the switchingoperation from the D2D communication to the cellular communication andalso to a case that the D2D communication is performed by using the DLradio resource.

As shown in FIG. 22, in an initial state, each of the UE 100-1 and theUE 100-2 performs the cellular communication (the UL communication andthe DL communication) in a cell of the eNB 200. Here, an operationenvironment is supposed where the UE 100-1 and the UE 100-2 are locatedfar from the eNB 200. In such an operation environment, the UL subframehandled by each of the UE 100-1 and the UE 100-2 is located before, on atime axis, the DL subframe.

In such a situation, in a case that the UE 100-1 and the UE 100-2 switchfrom the cellular communication to the D2D communication, in the UE100-2 (reception-side UE), a situation occurs that a DL subframe F61 inwhich the data reception of the cellular communication should beperformed and a UL subframe F62 in which the data reception of the D2Dcommunication should be performed overlap on a time axis, resulting in asituation where the simultaneous cellular and D2D reception isperformed. In FIG. 22, a rear-side portion of the DL subframe F61 inwhich the data reception of the cellular communication should beperformed overlaps a front-side portion of the UL subframe F62 in whichthe data reception of the D2D communication should be performed.

Thus, in the third embodiment, The eNB 200 configured to assign theradio resource to the UE 100-2 assigns the radio resource correspondingto one subframe, out of the overlapping two subframes, to the UE 100-2and assigns, to the UE 100-2, the radio resource corresponding to thenon-overlapping interval included in the other subframe for transmittingor receiving the Discovery signal.

In FIG. 22, the eNB 200 assigns to the UE 100-2 the radio resourcecorresponding to the DL subframe F61 in which the data reception of thecellular communication should be performed, out of the overlapping twosubframes. Further, the eNB 200 assigns, in unit of symbol, to the UE100-2 the radio resource corresponding to the non-overlapping intervalincluded in the UL subframe F62 in which the data reception of the D2Dcommunication should be performed, out of the overlapping two subframes.

As a result, in the UL subframe F62 in which the data reception of theD2D communication should be performed, the overlapping intervaloverlapping the DL subframe F61 in which the data reception of thecellular communication should be performed is set as the switching guardtime (interval having no data). Further, in the UL subframe F62 in whichthe data reception of the D2D communication should be performed, thenon-overlapping interval not overlapping the DL subframe F61 in whichthe data reception of the cellular communication should be performed isset as the interval in which the Discovery signal is transmitted orreceived.

It is noted that to notify the UE 100-1 and the UE 100-2 of theswitching guard time, the eNB 200 may transmit to the UE 100-1 and theUE 100-2 symbol identification information indicating each symbolconfiguring the switching guard time, and may also transmit to the UE100-1 and the UE 100-2 format identification information indicating asetting format for the switching guard time.

FIG. 23 is a sequence diagram showing the switching operation from thecellular communication to the D2D communication in a case of performingthe D2D communication by using the UL radio resource. In FIG. 23, asituation is supposed where the UE 100-1 and the UE 100-2 are adjacentto each other and the UE 100-1 and the UE 100-2 perform the cellularcommunication therebetween through a network. Further, a situation issupposed where a UE 100-3 and a UE 100-4 are located around the UE 100-1and the UE 100-2.

As shown in FIG. 23, in step S301, each of the UE 100-1 and the UE 100-2performs the cellular communication with the eNB 200. The eNB 200assigns to each of the UE 100-1 and the UE 100-2 the radio resource forthe cellular communication.

In step S302, the eNB 200 determines that the UE 100-1 and the UE 100-2are switched from the cellular communication to the D2D communication.

In step S303, the eNB 200 confirms whether or not the UE 100-2 that is areception-side UE in the D2D communication supports the simultaneousreception of cellular and D2D.

When the UE 100-2 does not support the simultaneous reception ofcellular and D2D, in step S304, the eNB 200 calculate, in unit ofsymbol, the switching guard time on the basis of the timing advancevalue set to the UE 100-2. For example, the eNB 200 acquires theaccumulated value of the timing advance value to determine on the basisof the accumulated value how far the UL subframe and the DL subframeoverlap in the UE 100-2.

In step S305, the eNB 200 notifies, in unit of subframe, the UE 100-1 tothe UE 100-4 of the switching timing from the cellular communication tothe D2D communication.

In step S306, the eNB 200 notifies the UE 100-1, the UE 100-3, and theUE 100-4 of the switching guard time and the transmission timing ofDiscovery signal.

In step S307 to S309, each of the UE 100-1, the UE 100-3, and the UE100-4 transmits the Discovery signal at the Discovery signaltransmission timing.

In step S310, the UE 100-1 and the UE 100-2 starts the D2D communicationafter the switching guard time and the Discovery signal transmissiontiming pass.

In step S311, each of the UE 100-1 to UE 100-4 notifies the eNB 200 of areception result of the Discovery signal.

[Other Embodiments]

In each of the above-described embodiments, the eNB 200 confirms whetheror not the simultaneous reception of cellular and D2D or thesimultaneous transmission of cellular and D2D is supported. However,when the simultaneous reception of cellular and D2D or the simultaneoustransmission of cellular and D2D transmission is performed, acommunication quality may deteriorate due to interference. Thus, the eNB200 may set the switching guard time on the basis of the timing advancevalue without confirming whether or not the simultaneous reception ofcellular and D2D or the simultaneous transmission of cellular and D2D issupported.

In each of the above-described embodiments, the switching guard time iscalculated and notified, for example, in the eNB 200; however, theswitching guard time may be calculated and notified, for example, in theUE 100-1 or the UE 100-2.

In the above-described each embodiment, the LTE system as one example ofa cellular system is described; however, the present invention is notlimited to the LTE system, and the present invention may be applied to acommunication system other than the LTE system.

In addition, the entire content of Japanese Patent Application No.2013-135605 (filed on Jun. 27, 2013) is incorporated in the presentspecification by reference.

INDUSTRIAL APPLICABILITY

The present invention is useful in a mobile communication filed.

The invention claimed is:
 1. A mobile communication system in which an uplink radio resource of cellular communication includes a plurality of uplink subframes divided on a time axis and a downlink radio resource of the cellular communication includes a plurality of downlink subframes divided on a time axis, comprising: a user terminal that performs D2D communication that is direct device-to-device communication by using the uplink radio resource, wherein when the user terminal switches communication modes between the cellular communication and the D2D communication, if the downlink subframe in which data reception of the cellular communication should be performed and the uplink subframe in which data reception of the D2D communication should be performed at least partially overlap on a time axis: the base station assigns a first radio resource corresponding to one subframe, out of the overlapping two subframes, to the user terminal, and assigns a second radio resource corresponding to the other subframe, out of the overlapping two subframes, to a user terminal other than the user terminal, and the user terminal performs data reception in the one subframe, out of the overlapping two subframes.
 2. A user terminal configured to perform D2D communication that is direct device-to-device communication by using an uplink radio resource in a mobile communication system in which the uplink radio resource of cellular communication includes a plurality of uplink subframes divided on a time axis, and a downlink radio resource of the cellular communication includes a plurality of downlink subframes divided on a time axis, the user terminal comprising: a controller configured to, when communication modes are switched between the cellular communication and the D2D communication, if the downlink subframe in which data reception of the cellular communication should be performed and the uplink subframe in which data reception of the D2D communication should be performed at least partially overlap on a time axis: receive, from a base station, a radio resource assignment, wherein the radio resource assignment assigns a first radio resource corresponding to one subframe, out of the overlapping two subframes, to the user terminal, and assigns a second radio resource corresponding to the other subframe, out of the overlapping two subframes, to a user terminal other than the user terminal; and perform data reception in the one subframe, out of the overlapping two subframes.
 3. A user terminal, comprising: a controller including a processor; a memory communicatively coupled to the processor; a transmitter; and a receiver, wherein the controller is configured to control performing Device to Device (D2D) communication that is direct device-to-device communication, the receiver is configured to receive, from a base station, first information indicating radio resources to be used for the user terminal to perform a transmission of the D2D communication, the transmitter is configured to transmit, to the base station, second information indicating that the user terminal supports simultaneous reception of downlink communication and the D2D communication, and the transmitter is further configured to transmit, to the base station, third information indicating that the user terminal supports simultaneous transmission of uplink communication and the D2D communication.
 4. The user terminal according to claim 3, wherein the transmitter is further configured to perform the uplink communication without the D2D communication when first time for the uplink communication and second time for the D2D communication at least partially overlap.
 5. A processor for controlling a user terminal, comprising: a memory communicatively coupled to the processor and including instructions, such that when executed by the processor performs the processes of: performing Device to Device (D2D) communication that is direct device-to-device communication, receiving, from a base station, first information indicating radio resources to be used for the user terminal to perform a transmission of the D2D communication, transmitting, to the base station, second information indicating that the user terminal supports simultaneous reception of downlink communication and the D2D communication, and transmitting, to the base station, third information indicating that the user terminal supports simultaneous transmission of uplink communication and the D2D communication. 